Conventionally, there is a technique proposed for identifying a person and authenticating the person in accordance with a physical characteristic of a human body. A physical characteristic obtained by an image taking device is compared with a physical characteristic that is registered in advance, so that the personal identification or the personal authentication is performed.
Image taking devices can be divided broadly into a contact type in which the device touches the human body and a noncontact type in which the device does not touch the human body. A noncontact type is desired in a place where an indefinite number of people uses from viewpoints of resistance to soiling, a hygienic aspect and psychological inhibition.
For example, if a contact type device is used for security of a facility, it is necessary to check every time whether or not a person who tries to enters the facility is permitted to enter. Namely, the device is frequently touched by people. Therefore, a glass face that is touched with a human skin may become dirty. As a result, the image taking process cannot be performed appropriately, and a correct result may not be obtained. For this reason, a noncontact type device is desired in a place where the authentication is performed frequently.
Similarly in the case where it is used for a control of entering and leaving a facility, image taking process should be performed ever time when a person enters or leaves the facility. In this case too, the contact type device is touched by people frequently. Therefore, a glass face that is touched with a hand may become dirty, so that the image taking process cannot be performed appropriately. In addition, a hygienic problem or a psychological inhibition may be caused about touching the dirty face. For this reason, a noncontact type device is desired.
A noncontact type is preferred to a contact type also in the case where it is used in a place with a strict hygienic rule such as a medical institute or a research institute. In addition, needs of noncontact products are increasing recently as a social trend as understood from that various antibacterial goods and hygienic goods have becomes hit goods from viewpoints of a hygienic problem or a psychological inhibition. In the case of taking an image of a moving object, the contact type device cannot be used for such purpose.
However, it is difficult to set a subject at the same position by the noncontact type device every time of taking an image. Therefore, luminance of an obtained image may vary every image taking process. In this case, it may be difficult to authenticate a person because of a mismatch between a pattern of the obtained image and a pattern of the image that was taken in advance.
In addition, in the noncontact type device, a portion except the subject, i.e. a background portion may be included in the image. Then, it may be difficult to authenticate a person because of a mismatch between a pattern of the obtained image and a pattern of the image that was taken in advance.
There are proposed some methods for deleting a background portion, but each of them has a problem. For example, the method described in the patent document 1 below obtains movement information of an image by a differential between frames and accumulates the same, thereby determines that a pixel without any movement in a past predetermined period belongs to the background area for obtaining the background image. However, it is difficult in this method to distinguish the object from the background when the object having a uniform color, e.g., a white sheet of paper passes the front of a camera. The method described in the patent document 2 has a similar problem.
The method described in the patent document 3 detects a depth of an image so that a foreground is separated from a background in accordance with the depth. However, this method needs a device for detecting the depth that will be a large scale and expensive. The method described in the patent document 4 has a similar problem.
An object of the present invention is to solve such problems by providing a noncontact type image taking device that can take an image of high accuracy easily.
Patent Document 1:
Japanese unexamined patent publication No. 7-284086
Patent Document 2
Japanese unexamined patent publication No. 2002-150294
Patent Document 3
Japanese unexamined patent publication No. 5-95509
Patent Document 4
Japanese unexamined patent publication No. 2001-137241